This invention relates to a process for the preparation of polyethylene having a broad molecular weight distribution.
It is well known that linear polyethylene may be prepared by the polymerization of ethylene (optionally with one or more olefins or diolefins such as butene, hexene, octene or hexadiene) using a xe2x80x9cZieglerxe2x80x9d catalyst system which comprises a transition metal compound (such as a titanium halide) or a vanadium halide and an aluminum alkyl. Polyethylene produced in this manner, particularly xe2x80x9clinear low density polyethylenexe2x80x9d, is in widespread commercial use. However, the conventional linear low density polyethylene (xe2x80x9clldpexe2x80x9d) made with Ziegler catalysts suffers from a number of deficiencies. Most notably, conventional lldpe is a heterogeneous product which contains a small fraction of low molecular weight wax and a comparatively large amount of very high molecular weight homopolymer. The heterogeneous nature of these polymers generally detracts from the physical properties of finished goods made from them.
Accordingly, a great deal of effort has been directed towards the preparation of xe2x80x9chomogeneousxe2x80x9d lldpe resins which mitigate this problem. In particular, it is now well known to those skilled in the art that so-called xe2x80x9cmetallocenexe2x80x9d catalysts may be used to produce homogeneous lldpe resin. These homogeneous resins are, however, not without problems. Most notably, these homogeneous resins typically have a narrow molecular weight distribution and are difficult to xe2x80x9cprocessxe2x80x9d or convert into finished polyethylene products. Thus, efforts to improve the processability of homogeneous polyethylene resin by broadening the molecular weight distribution are disclosed in the art.
One approach which has been used to achieve this object is the use of mixed catalyst systems in a single reactor. For example, U.S. Pat. No. 4,530,914 (Ewen et al., to Exxon) teach the preparation of xe2x80x9cbroadxe2x80x9d polymers through the use of two different metallocene catalysts and U.S. Pat. No. 4,701,432 (Welborn, to Exxon) teaches the use of a supported catalyst prepared with a metallocene catalyst and a Ziegler Natta catalyst. Many others have subsequently attempted to use similar mixed catalyst systems, as described in U.S. Pat. Nos. 5,767,031; 5,594,078; 5,648,428; 4,659,685; 5,145,818; 5,395,810; and 5,614,456.
However, the use of xe2x80x9cmixedxe2x80x9d catalyst systems is generally associated with operability problems. For example, the use of two catalysts on a single support (as taught by Welborn in U.S. Pat. No. 4,701,432) may be associated with a reduced degree of process control flexibility (e.g. if the polymerization reaction is not proceeding as desired when using such a catalyst system, then it is difficult to establish which corrective action should be taken because the corrective action will typically have a different effect on each of the two different catalyst components). Moreover, the two different catalyst/cocatalyst systems may interfere with one anotherxe2x80x94for example, the organoaluminum component which is often used in Ziegler Natta or chromium catalyst systems may xe2x80x9cpoisonxe2x80x9d a metallocene catalyst.
Another alternative is to use two different homogeneous catalysts in two different polymerization reactors. In commonly assigned U.S. Pat. No. 6,063,879 (Stephan et al.) there are disclosed certain phosphinimine catalysts which may be used to produce homogeneous polyethylene. The use of such phosphinimine catalysts in a xe2x80x9cdual reactorxe2x80x9d polymerization system to prepare polymers having a broad molecular weight distribution is also disclosed in a commonly assigned patent application (Brown et al., U.S. Ser. No. 09/364,703 and corresponding Canadian application 2,245,375).
Each of the approaches to produce broad MWD polymers requires the use of at least two distinct polymerization catalysts or two distinct polymerization reactors.
We have now discovered a process to prepare broad MWD ethylene polymers in a single polymerization reactor using a single phosphinimine catalyst and a specific activation system.
The present process is simpler/more elegant than the above described prior art process in the sense that the present process does not require the use of two catalysts and/or two reactors. The simplicity of the present process offers the potential to improve process control and reduce costs in comparison to the prior art processes.
A process for preparing thermoplastic ethylene alpha olefin copolymer having a polydispersity (or molecular weight distribution, Mw/Mn) of greater than 2, said process comprising polymerizing ethylene and at least one other C3 to 10 alpha olefin under medium pressure solution polymerization conditions at a temperature of greater than 170xc2x0 C. to 300xc2x0 C. in the presence of a catalyst system comprising:
1) an organometallic catalyst comprising a group 4 metal, at least one phosphinimine ligand and at least one activatable ligand;
2) a four coordinate boron activator; and
3) at least one trialkyl aluminum.
The catalyst used in this invention is an organometallic complex of a group 4 metal having at least one phosphinimine ligand and at least one activatable ligand. Highly preferred catalysts also contain a cyclopentadienyl ligand.
The preferred phosphinimine catalysts used in this invention are defined by the formula:
(Cp)aM(Pl)b(L)c
wherein Pl is a phosphinimine ligand (see section 1.1 below); Cp is a cyclopentadienyl-type ligand (section 1.2 below); L is an activatable ligand (section 1.3 below); M is a metal selected from Ti, Hf and Zr; and wherein a is 0 or 1; b is 1 or 2; a+b=2; c is 1 or 2; and a+b+c=the valence of the metal M.
The most preferred catalysts are those in which the metal is 4 valent. For example, a catalyst may be a cyclopentadienyl-phosphinimine complex of titanium, zirconium, or hafnium having two additional, monoanionic ligands. It is particularly preferred that each catalyst contains one phosphinimine ligand, one cyclopentadienyl ligand and two chloride or alkyl ligands.
Each catalyst must contain at least one phosphinimine ligand which is covalently bonded to the metal. Phosphinimine ligands are defined by the formula: 
wherein each R1 is independently selected from the group consisting of a hydrogen atom, a halogen atom, C1-20 hydrocarbyl radicals which are unsubstituted by or further substituted by a halogen atom, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical, a silyl radical of the formula:
xe2x80x94Sixe2x80x94(R2)3
wherein each R2 is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical, C6-10 aryl or aryloxy radicals, and a germanyl radical of the formula:
Gexe2x80x94(R2)3
wherein R2 is as defined above.
The preferred phosphinimines are those in which each R1 is a hydrocarbyl radical. A particularly preferred phosphinimine is tri-(tertiary butyl) phosphinimine (i.e. where each R1 is a tertiary butyl group).
As used herein, the term cyclopentadienyl-type ligand is meant to convey its conventional meaning, namely a ligand having a five carbon ring which is bonded to the metal via eta-5 bonding. Thus, the term xe2x80x9ccyclopentadienyl-typexe2x80x9d includes unsubstituted cyclopentadienyl, substituted cyclopentadienyl, unsubstituted indenyl, substituted indenyl, unsubstituted fluorenyl and substituted fluorenyl. An exemplary list of substituents for a cyclopentadienyl ligand includes the group consisting of C1-10 hydrocarbyl radical (which hydrocarbyl substituents are unsubstituted or further substituted); a halogen atom, C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical; an amido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; silyl radicals of the formula xe2x80x94Sixe2x80x94(R)3 wherein each R is independently selected from the group consisting of hydrogen, a C1-8 alkyl or alkoxy radical C6-10 aryl or aryloxy radicals; germanyl radicals of the formula Gexe2x80x94(R)3 wherein R is as defined directly above.
The term xe2x80x9cactivatable ligandxe2x80x9d refers to a ligand which may be activated by a cocatalyst, (or xe2x80x9cactivatorxe2x80x9d), to facilitate olefin polymerization. Exemplary activatable ligands are independently selected from the group consisting of a hydrogen atom, a halogen atom, a C1-10 hydrocarbyl radical, a C1-10 alkoxy radical, a C5-10 aryl oxide radical; each of which said hydrocarbyl, alkoxy, and aryl oxide radicals may be unsubstituted by or further substituted by a halogen atom, a C1-8 alkyl radical, a C1-8 alkoxy radical, a C6-10 aryl or aryloxy radical, an amido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals; a phosphido radical which is unsubstituted or substituted by up to two C1-8 alkyl radicals.
The number of activatable ligands depends upon the valency of the metal and the valency of the activatable ligand. The catalyst metals are group 4 metals, preferably in their highest oxidation state (i.e. 4+) and the preferred activatable ligands are monoanionic (such as a halidexe2x80x94especially chloride or an alkylxe2x80x94especially methyl). Thus, the preferred catalyst contains a phosphinimine ligand, a cyclopentadienyl ligand and two chloride (or methyl) ligands bonded to the group 4 metal. In some instances, the metal of the catalyst component may not be in the highest oxidation state. For example, a titanium (III) component would contain only one activatable ligand. Also, it is permitted to use a dianionic activatable ligand (such as a butadienyl ligand) although this is not preferred.
The catalyst component described in part 1 above is used in combination with a boron activator to form an active catalyst system for olefin polymerization as described in more detail below.
So-called boron activators (also known as xe2x80x9cionic activatorsxe2x80x9d are well known for use with metallocene catalysts. See, for example, U.S. Pat. No. 5,198,401 (Hlatky and Turner) and U.S. Pat. No. 5,132,380 (Stevens and Neithamer).
Whilst not wishing to be bound by any theory, it is thought by many of those skilled in the art that boron activators initially cause the abstraction of one or more of the activatable ligands in a manner which ionizes the catalyst into a cation, then provides a bulky, labile, non-coordinating anion which stabilizes the catalyst in a cationic form. The resulting bulky, non-coordinating anion permits olefin polymerization to proceed at the cationic catalyst center (presumably because the non-coordinating anion is sufficiently labile to be displaced by monomer which coordinates to the catalyst. It should be expressly noted that the boron activator/phosphinimine catalyst may also form a non-ionic coordination complex which is catalytically active for olefin polymerization. The boron activator is described as being four coordinatexe2x80x94i.e. there must be four ligands bonded to the boron atom. Preferred boron activators are described in (i)-(ii) below:
(i) compounds of the formula [R5]+[B(R7)4]xe2x88x92 wherein B is a boron atom, R5 is a aromatic hydrocarbyl (e.g. triphenyl methyl cation) and each R7 is independently selected from the group consisting of phenyl radicals which are unsubstituted or substituted with from 3 to 5 substituents selected from the group consisting of a fluorine atom, a C1-4 alkyl or alkoxy radical which is unsubstituted or substituted by a fluorine atom; and a silyl radical of the formula xe2x80x94Sixe2x80x94(R9)3; wherein each R9 is independently selected from the group consisting of a hydrogen atom and a C1-4 alkyl radical; and
(ii) compounds of the formula [(R8)tZH]+[B(R7)4]xe2x88x92 wherein B is a boron atom, H is a hydrogen atom, Z is a nitrogen atom or phosphorus atom, t is 2 or 3 and R8 is selected from the group consisting of C1-8 alkyl radicals, a phenyl radical which is unsubstituted or substituted by up to three C1-4 alkyl radicals, or one R8 taken together with the nitrogen atom may form an anilinium radical and R7 is as defined above.
In the above compounds preferably R7 is a pentafluorophenyl radical. In general, preferred boron activators may be described as salts of tetra(perfluorophenyl) boron. More specifically, the preferred activators are anilinium, carbonium, oxonium, phosphonium and sulfonium salts of tetra(perfluorophenyl) boron, with anilinium and trityl (or xe2x80x9ctriphenyl methyliumxe2x80x9d) salts being especially preferred.
It should also be noted that three coordinate boron activators (i.e. compounds of the formula B(R7)3 where R7 is as defined above) are not suitable for use in the process of this invention. This is surprising as such compounds are well known as activators for metallocene catalysts. However, for reasons which are not completely understood, the use of a trivalent boron activator is not suitable for preparing polymers having a broad molecular distribution in accordance with the process of this invention.
Exemplary ionic activators include:
triethylammonium tetra(phenyl)boron,
tripropylammonium tetra(phenyl)boron,
tri(n-butyl)ammonium tetra(phenyl)boron,
trimethylammonium tetra(p-tolyl)boron,
trimethylammonium tetra(o-tolyl)boron,
tributylammonium tetra(pentafluorophenyl)boron,
tripropylammonium tetra(o,p-dimethylphenyl)boron,
tributylammonium tetra(m, m-dimethylphenyl)boron,
tributylammonium tetra(p-trifluoromethylphenyl)boron,
tributylammonium tetra(pentafluorophenyl)boron,
tri(n-butyl)ammonium tetra(o-tolyl)boron,
N,N-dimethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)boron,
N,N-diethylanilinium tetra(phenyl)n-butylboron,
N,N-2,4,6-pentamethylanilinium tetra(phenyl)boron,
di-(isopropyl)ammonium tetra(pentafluorophenyl)boron,
dicyclohexylammonium tetra(phenyl)boron,
triphenylphosphonium tetra(phenyl)boron,
tri(methylphenyl)phosphonium tetra(phenyl)boron,
tri(dimethylphenyl)phosphonium tetra(phenyl)boron,
tropillium tetrakispentafluorophenyl borate,
triphenylmethylium tetrakispentafluorophenyl borate,
benzene (diazonium) tetrakispentafluorophenyl borate,
tropillium tetrakis (2,3,5,6-tetrafluorophenyl) borate,
triphenylmethylium tetrakis (2,3,5,6-tetrafluorophenyl) borate,
benzene (diazonium) tetrakis (3,4,5-trifluorophenyl) borate,
tropillium tetrakis (3,4,5-trifluorophenyl) borate,
benzene (diazonium) tetrakis (3,4,5-trifluorophenyl) borate,
tropillium tetrakis (1,2,2-trifluoroethenyl) borate,
triphenylmethylium tetrakis (1,2,2-trifluoroethenyl) borate,
benzene (diazonium) tetrakis (1,2,2-trifluoroethenyl) borate,
tropillium tetrakis (2,3,4,5-tetrafluorophenyl) borate,
triphenylmethylium tetrakis (2,3,4,5-tetrafluorophenyl) borate, and
benzene (diazonium) tetrakis (2,3,4,5-tetrafluorophenyl) borate.
Readily commercially available ionic activators which are suitable for the process of this invention are:
N,N-dimethylaniliniumtetrakispentafluorophenyl borate, and
triphenylmethylium tetrakispentafluorophenyl borate (also known as xe2x80x9ctrityl boratexe2x80x9d).
It is preferred to use the boron activator in an equimolar amount with respect to the transition metal of the catalyst (i.e. boron/titanium ratio of 1/1, when the catalyst is an organotitanium complex) through mole ratios of from 0.3/1 to 10.0/1 may be used.
The use of a trialkyl aluminum alkyl is essential to the process of this invention. Preferred amounts are from 1:1 to 100:1 (on a molar basis, with reference to the group 4 metal contained in the phosphinimine catalyst).
Preferred aluminum alkyls are trimethyl aluminum, triethyl aluminum, tri isobutyl aluminum and tri n-butyl aluminum. Mixed aluminum alkyls (i.e. where different alkyl ligands are bonded to the aluminum) and/or mixtures of aluminum alkyls may also be used. However, the aluminum compound must be a trialkyl aluminum. Most notably, for reasons which are not completely understood, the use of an aluminum alkyl/alkoxide (i.e. an organoaluminum having an alkyl ligand and an alkoxy ligand) is not suitable because it does not always lend to polymers having a broadened molecular weight distribution (as is illustrated in the Examples).
The use of alumoxane (also known as aluminoxane) is optional.
The alumoxane may be of the formula:
(R4)2AIO(R4AIO)mAI(R4)2
wherein each R4 is independently selected from the group consisting of C1-20 hydrocarbyl radicals and m is from 0 to 50, preferably R4 is a C1-4 alkyl radical and m is from 5 to 30. Methylalumoxane (or xe2x80x9cMAOxe2x80x9d) in which each R is methyl is the preferred alumoxane.
Alumoxanes are well known as cocatalysts, particularly for metallocene-type catalysts. Alumoxanes are also readily available articles of commerce.
The use of an alumoxane cocatalyst generally requires a molar ratio of aluminum to the transition metal in the catalyst from 20:1 to 1000:1. Preferred ratios are from 50:1 to 250:1.
Many commercially available alumoxanes are known to contain trialkyl aluminum. Thus, such alumoxanes may be the source of the trialkyl aluminum which is essential to the process of this invention. This is described in more detail in the Examples.
Solution processes for the (co)polymerization of ethylene are well known in the art. These processes are conducted in the presence of an inert hydrocarbon solvent typically a C5-12 hydrocarbon which may be unsubstituted or substituted by a C1-4 alkyl group, such as pentane, methyl pentane, hexane, heptane, octane, cyclohexane, methylcyclohexane and hydrogenated naphtha. An example of a suitable solvent which is commercially available is xe2x80x9cIsopar Exe2x80x9d (C8-12 aliphatic solvent, Exxon Chemical Co.).
The polymerization temperature in a conventional solution process is from about 80xc2x0 C. to about 300xc2x0 C. (preferably from about 120xc2x0 C. to 250xc2x0 C.). However, as is illustrated in the Examples, the polymerization temperature for the process of this invention must be greater than about 160xc2x0 C. (because at lower polymerization temperature, the resulting polymers generally do not exhibit the broad molecular weight distribution which is a desirable feature of this invention). The upper temperature limit will be influenced by considerations which are well known to those skilled in the art, such as a desire to maximize operating temperature (so as to reduce solution viscosity). While still maintaining good polymer properties (as increased polymerization temperatures generally reduce the molecular weight of the polymer). In general, the upper polymerization temperature will preferably be between 200 and 300xc2x0 C. (especially 220 to 250xc2x0 C.). The most preferred reaction process is a xe2x80x9cmedium pressure processxe2x80x9d, meaning that the pressure in the reactor is preferably less than about 6,000 psi (about 42,000 kiloPascals or kPa). Preferred pressures are from 10,000 to 40,000 kPa, most preferably from about 2,000 psi to 3,000 psi (about 14,000-22,000 kPa).
Suitable monomers for copolymerization with ethylene include C3-20 mono- and di-olefins. Preferred comonomers include C3-12 alpha olefins which are unsubstituted or substituted by up to two C1-6 alkyl radicals, C8-12 vinyl aromatic monomers which are unsubstituted or substituted by up to two substituents selected from the group consisting of C1-4 alkyl radicals, C4-12 straight chained or cyclic diolefins which are unsubstituted or substituted by a C1-4 alkyl radical. Illustrative non-limiting examples of such alpha-olefins are one or more of propylene, 1-butene, 1-pentene, 1-hexene, 1-octene and 1-decene, styrene, alpha methyl styrene, and the constrained-ring cyclic olefins such as cyclobutene, cyclopentene, dicyclopentadiene norbornene, alkyl-substituted norbornes, alkenyl-substituted norbornes and the like (e.g. 5-methylene-2-norbornene and 5-ethylidene-2-norbornene, bicyclo-(2,2,1)-hepta-2,5-diene).
The polyethylene polymers which may be prepared in accordance with the present invention are LLDPE""s which typically comprise not less than 60, preferably not less than 75 weight % of ethylene and the balance one or more C4-10 alpha olefins, preferably selected from the group consisting of 1-butene, 1-hexene and 1-octene. The polyethylene prepared in accordance with the present invention may be LLDPE having a density from about 0.910 to 0.935 g/cc or (linear) high density polyethylene having a density above 0.935 g/cc. The present invention might also be useful to prepare polyethylene having a density below 0.910 g/ccxe2x80x94the so-called very low and ultra low density polyethylenes.
Generally the alpha olefin may be present in an amount from about 3 to 30 weight %, preferably from about 4 to 25 weight %.
The present invention may also be used to prepare co- and ter-polymers of ethylene, propylene and optionally one or more diene monomers. Generally, such polymers will contain about 50 to about 75 weight % ethylene, preferably about 50 to 60 weight % ethylene and correspondingly from 50 to 25 weight % of propylene. A portion of the monomers, typically the propylene monomer, may be replaced by a conjugated diolefin. The diolefin may be present in amounts up to 10 weight % of the polymer although typically is present in amounts from about 3 to 5 weight %. The resulting polymer may have a composition comprising from 40 to 75 weight % of ethylene, from 50 to 15 weight % of propylene and up to 10 weight % of a diene monomer to provide 100 weight % of the polymer. Preferred but not limiting examples of the dienes are dicyclopentadiene, 1,4-hexadiene, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene and 5-vinyl-2-norbornene, especially 5-ethylidene-2-norbornene and 1,4-hexadiene.
The monomers are dissolved/dispersed in the solvent either prior to being fed to the reactor (or for gaseous monomers the monomer may be fed to the reactor so that it will dissolve in the reaction mixture). Prior to mixing, the solvent and monomers are generally purified to remove potential catalyst poisons such as water, oxygen or metal impurities. The feedstock purification follows standard practices in the art, e.g. molecular sieves, alumina beds and oxygen removal catalysts are used for the purification of monomers. The solvent itself as well (e.g. methyl pentane, cyclohexane, hexane or toluene) is preferably treated in a similar manner.
The feedstock may be heated or cooled prior to feeding to the reactor.
Generally, the catalyst components may be premixed in the solvent for the reaction or fed as separate streams to the reactor. In some instances premixing it may be desirable to provide a reaction time for the catalyst components prior to entering the reaction. Such an xe2x80x9cin line mixingxe2x80x9d technique is described in a number of patents in the name of DuPont Canada Inc (e.g. U.S. Pat. No. 5,589,555 issued Dec. 31, 1996).